fix(commit): respect daylight saving when computing utc-offset

Related to #362

Author: Byron

Diff for: git/ext/gitdb

@@ -27,7 +27,10 @@
 
 from time import (
     time,
-    altzone
+    daylight,
+    altzone,
+    timezone,
+    localtime
 )
 import os
 from io import BytesIO
@@ -327,7 +330,8 @@ def create_from_tree(cls, repo, tree, message, parent_commits=None, head=False,
 
         # PARSE THE DATES
         unix_time = int(time())
-        offset = altzone
+        is_dst = daylight and localtime().tm_isdst > 0
+        offset = altzone if is_dst else timezone
 
         author_date_str = env.get(cls.env_author_date, '')
         if author_date: